1. Field of the Invention
The present invention relates to a method for determining a power limiting value for an electric machine in a vehicle, in particular in an electric or hybrid vehicle, a computer program, and a control unit for controlling an electric machine in a vehicle.
2. Description of the Related Art
Electric machines are used, for example, in hybrid vehicles, where they are operated either in motor mode or in generator mode. In the motor mode, the electric machine is supplied with power from an energy storage device such as a battery, and generates an additional drive torque, which supports an internal combustion engine, for example, in an acceleration phase. In the generator mode, the electric machine generates electric power, which is stored in the energy storage device. The operating mode and power of the electric machine are set with the aid of an inverter.
In order to make the best possible use of the power capability of the electric machine on the one hand and to prevent damage to the energy storage device on the other hand, a power limiting value must be observed in both the motor mode and the generator mode of the electric machine; when this power limiting value is reached, the inverter reduces the power of the electric machine, and this power limiting value is essentially determined by a maximally allowable battery power. The maximally possible battery power in each case may be specified, for example, via a maximally allowable current at a predefined voltage. However, in order to precisely determine the maximally possible battery power, the efficiency of the energy storage device must be taken into account. This efficiency, however, is influenced by many factors such as operating point, temperatures, etc., and is therefore difficult to determine mathematically with sufficient accuracy. If this accuracy is not achieved, a power limit set excessively low causes the power capability of the electric machine not to be fully utilized. Conversely, a power limit set excessively high may permanently damage the energy storage device.